Mazda - B4000 :: 1994 - Parasitic Drain From Audio Amp?

I've traced a 0.3-amp parasitic drain to the audio power amp (fuse #16, 20A). Pulling the fuse, the radio still works (it's on fuse #15) but then there's no sound from the speakers. (There's also no current drain. I'm weighing if it's more important for the truck to run or to sound good.)

I replaced the radio a month ago. It was definitely not working right, and the new radio definitely is. With the #16 fuse in place, the sound out of front and back speakers was great. "Was" because if I leave the fuse in, the battery gets drained in a few days, so the fuse is now out.

Where the audio amp is. My Haynes manual doesn't say. The amp has no user controls. It just takes the radio's output and feeds audio to the front and back speakers. The #16 fuse is what gives the amp power. Some online posts say it's under the glove box, some say behind the rear right speaker. All agree it's a bear to access.

So a few questions:

Why the power amp would be drawing current even when the truck and radio are powered off?

How does one troubleshoot this, short of replacing the power amp, which of course I don't have an extra lying around and wouldn't want to replace anyway as it's otherwise working great.

What do you think of installing a switch on the audio power amp circuit so that when the truck is parked, I switch it off, and no current drain, but when the truck is driven, I switch it on, letting me hear the radio or CD. This is obviously a kludge/ghetto solution, but the advantage is that I don't have to access or replace the audio amp. I want to keep using the fuse, of course, so am looking for a way to wire a switch into the fuse box while keeping the fuse in the circuit. If I knew which wire goes between the fuse and the audio amp, I'd splice a switch there, but I don't, and again, the Haynes manual is not useful.

1994 B4000, extended cab, bought in 1996....